1. Field of the Invention
The present invention relates to a device for accurately measuring the flying height of a read/write head over a rotating disk or the like, and in particular, a white light interferometric device capable of measuring extremely small flying heights, all the way down to zero microinches.
2. Description of the Related Art
Read/write heads in magnetic storage devices are designed to "fly" over the storage medium. In Winchester-type hard drives, upon start-up of the drive, once the storage disk achieves a certain angular velocity, a cushion of circulating air above the surface of the disk forces the head up off the surface of the disk to thereby achieve a flying height. Having very low flying heights offers several advantages, primary among them is that flying the head very close to the disk surface allows for a high data bit density (i.e., the number of data bits per inch on a data track of the storage disk). Thus, there has been an industry wide push to decrease the height at which read/write heads are maintained over the recording surfaces. In the 1960's flying heights were commonly about 100 microinches (.mu."). At present, technological advances in read/write head and disk drive design have allowed the reduction of flying heights to as low as a fraction of a micron.
In order to design and effectively evaluate the operation and performance of a disk drive, it is necessary to determine precisely how high the head flies above the disk, and whether there is any significant variation in the flying height as the disk rotates. When head flying heights were larger, methods such as conventional interferometry using either white or monochromatic light were used to accurately measure the height.
In conventional white light interferometry, as shown in FIG. 1a, a white light source 15 (i.e., the spectrum of all visible frequency electromagnetic waves) is directed through, for example, the bottom of a glass disk 16. A first portion of the light 15a is reflected back down off the upper surface of the disk, while a second portion of the light 15b passes through the disk and is reflected back down off the lower surface of the air-bearing slider 17. While each of the plurality of wavelengths in both the first and second portions of light still have the same phase upon reflection, the phase of the first portion will have shifted with respect to the second portion. Thus, when the first and second portions of light recombine, they form an interference pattern. This interference pattern is visible to the human eye as including at least one color from the visible light spectrum (a common example of an interference pattern in another context is the one seen reflecting off an oil slick on pavement. The visible colors result from an interference pattern due to light waves recombining after bouncing off the top and bottom surfaces of the oil). The colors of the interference pattern resulting from light waves reflecting off the upper surface of a disk and air-bearing slider are uniquely indicative of the height of the air-bearing slider above the surface of the disk.
A problem with such a method is that, while the color of an interference pattern is a good estimation of the frequency of the light waves contained therein, it is extremely difficult with the naked eye to determine the actual frequencies of the many light waves in the pattern. Without known frequencies, the determination of flying height is merely an estimation. This problem has been solved by inputting the interference pattern into a spectrophotometer, which can accurately measure all of the frequencies present in the pattern. The spectrophotometer creates an intensity profile which may then be analyzed by computer algorithm to accurately determine flying height.
An example of such an interferometric flying height measuring device is manufactured by Pacific Precision Laboratories Inc. (PPL), 9207 Eton Avenue, Chatworth, Calif. 91311. The PPL device is capable of measuring flying heights of approximately 4-5 microinches (.mu."). However, it is extremely difficult with the PPL device and other conventional interferometric flying height measuring devices to measure flying heights smaller than that. With flying heights below 4-5 .mu.", the phase shift of the first portion of light reflecting off the upper surface of the disk is very slight with respect to the second portion of light reflecting off the air-bearing slider. This slight phase shift yields an interference pattern wherein the intensity of each wavelength is relatively weak, and the intensity profile of all the wavelengths together is a relatively flat curve. FIG. 1b shows several intensity profiles and the flying heights at which they are created. As can be seen, the curves below 4-5 .mu." are relatively flat and indistinguishable. It is thus very difficult to accurately determine a flying height from these profiles. Disk drives are presently being developed having flying heights smaller than 4-5 .mu." and conventional white light interferometric devices are incapable of accurately measuring these flying heights.
Another disadvantage to conventional interferometric measurement of flying heights is that the surface of the glass disk is not a good reflector, and the intensity of the light reflected off of the slider is much stronger than the intensity of the light reflected from the upper surface of the disk. This results in a weak interference pattern.
Monochromatic interferometry has also been used to measure head flying height. In such systems a monochromatic light wave, from a laser for example, is directed onto a glass disk and air-bearing slider and an interference pattern results as described above. Using a single frequency light source allows an easier measurement of the resulting interference pattern. While monochromatic interferometric measuring devices are capable of measuring extremely small flying heights, they are very expensive. An average cost of such a machine is approximately 5 to 10 times more expensive than white light interferometric flying height measuring devices. Moreover, monochromatic devices are difficult to calibrate and must be used in a highly controlled environment. All of these factors render monochromatic devices impractical for large scale use in production line testing of read/write head flying heights.